Hints of structure beyond the visible universe

Colossal structures larger than the visible universe - forged during the period of cosmic inflation nearly 14 billion years ago - may be responsible for a strange pattern seen in the big bang's afterglow, says a team of cosmologists. If confirmed, the structures could provide precious information about the universe's earliest moments.

In the first instant after its birth, the universe is thought to have experienced a rapid growth spurt called inflation. During this period, space itself expanded faster than the speed of light.

Inflation solves some cosmological puzzles, such as why relic radiation from the big bang, released when the universe was less than 400,000 years old, is relatively uniform.

Called the cosmic microwave background (CMB), the radiation can be observed in all directions in the sky. It has a slightly mottled appearance due to small differences in temperature from place to place in the early universe. The temperature differences are thought to be caused by variations in the density of matter, with denser regions being warmer than emptier regions.

But the theory of inflation predicts that the mottling should be equally prominent in all directions. Curiously, it is 10% more pronounced on one side of the sky than the other, an asymmetry that was reported in 2004 by Hans Eriksen of the University of Oslo in Norway, and colleagues. Their study was based on a map made by the Wilkinson Microwave Anisotropy Probe (WMAP) satellite.

Now, cosmologists led by Adrienne Erickcek of Caltech in Pasadena, US, think they may have found the reason for this pattern. They suggest the universe has been skewed by the imprint of primordial structures that date back to the period of inflation.

Extra field

The structures stretch beyond the edge of the observable universe, which is essentially confined to a region with a radius of 14 billion light years, since only light from within that distance has had time to reach us since the big bang. The entire "global" universe is about 10100 times as large as the universe we can see.

In the inflation scenario, the expansion is driven by an energy field of still-mysterious origin. Erickcek and her colleagues argue that the asymmetry could be the remnant of fluctuations in an additional energy field, which started out very tiny, but were blown up by inflation until they were larger than the observable universe.

As a result, the value of this energy field varied from one side of the universe to the other at early times, enhancing the variations in temperature - and matter density - on one side of the sky relative to the other.

The conclusion, if correct, would shatter a cherished assumption about the universe. "One of the basic tenets of cosmology is that the universe is the same in all directions, and the standard model of inflation is built on that foundation," Erickcek told New Scientist. "If the asymmetry is real, then it tells us that one side of our universe is somehow different than the other side."

Few and far between

The asymmetry could theoretically be explained by the standard, single-field inflation scenario, though researchers say that is unlikely. That's because the standard theory allows for a few large-scale differences across vast regions of space. But these asymmetrical spots are expected to be few and far between, meaning that there is only a 1% chance that our observable universe would happen to occupy one.

Charles Bennett of Johns Hopkins University in Baltimore, Maryland, US, the chief scientist for the WMAP mission, notes that Erickcek's team has predicted additional subtle differences in the CMB compared to the standard inflation picture.

These predictions could be tested when more sensitive CMB maps are available. "It might be within reach of WMAP, and it will be within reach of Planck," he told New Scientist, referring to the European Space Agency satellite scheduled to launch later in 2008 to scrutinise the CMB.

Mysterious era

If further observations bear out the scenario, it would provide some precious new information about the universe's earliest moments, about which little is known. "It was a period of extremely rapid expansion, but what drove that expansion and how long it lasted is an open question," Erickcek says.

Alan Guth of MIT in Cambridge, Massachusetts, US, one of the scientists who pioneered the inflation idea in the 1980s, says inflation is only the framework of a theory, with many details remaining to be filled in.

He says the team's "well thought-out" analysis is just the kind of thing needed to help do that. "Although the hint [from asymmetry] may very well turn out to be a fluke, it is only by pursuing such hints that new ideas will be generated, and that ultimately we will have a chance to find the right theory," he told New Scientist.

The asymmetry is distinct from another curious pattern in the CMB called the axis of evil, which some scientists have proposed is also a remnant of exotic physics during inflation.

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Subtle signs of a skewed universe are present in the WMAP satellite's map of radiation left over from the big bang, a new study says (Image: NASA/WMAP Science Team)